1. Field of the Invention
The present invention relates to a laser welding method which can be used for laser-welding aluminum alloy parts.
2. Description of the Prior Art
As one example of manufacturing automobile parts of aluminum alloy, the manufacture of aluminum alloy automobile hoods is taken. In the manufacture, an inner panel having a peripheral portion and an outer panel having a hemmed portion are welded at the hemmed portion. If a conventional pressure-type spot welding is applied, the hemmed portion will be seriously deformed under the pressurizing forces from welding tips. This means that it is not desirable to use the conventional spot welding for welding aluminum alloy parts. In place of conventional spot welding, MIG welding or TIG welding could be used. However, because the thermal conductivity and the thermal expansion rate of the aluminum alloy are much greater than those of steel, relatively large deformations will be generated in the weld deposits and the heat-affected zone, and will need repair after welding.
To suppress deformation, the utility of laser welding of aluminum alloy parts is being investigated. For example, Publication, "NIKKEI MECHANICAL" published on May 3, 1993 from NIKKEI BP Company Ltd. discloses the application of laser welding to automobile hoods. In the laser welding, a Yttrium-Aluminum-Garnet (hereinafter, YAG) laser is used and is transmitted from a laser generator to a laser torch via optical fibers. The laser torch is moved along a welding schedule line to make intermittent weld beads each having a length of about 20 mm. The reason for using a YAG laser is that a YAG laser has a wave length of 1.06 micron and is almost not absorbed by the optical fibers constructed of SiO.sub.2.
However, with the above-described intermittent weld beads there is a problem that the thermal expansion and shrinkage caused in the weld beads and heat-affected zone thereof are relatively large so that relatively large deformations are still caused in the welding portion of the parts.